This present invention relates to a new method to prepare and sterilize human skin allografts for use as biological dressings. This method has applications in wound and burn therapy.
By way of background, allograft skin has been shown to provide an excellent temporary skin coverage for burn patients, acting as a biological dressing. Allograft skin protects the wound from desiccation, contamination, and decreases wound pain. When allograft skin shows general adherence to a burn wound and evidence of graft vascularization within 48 to 72 hours of application, one can anticipate an excellent take of autograft skin applied to the wound following removal of the allograft skin. Limitations of fresh allograft skin includes the dearth of material, the need for refrigerated storage facilities, and a limited "effective" shelf life of approximately seven to ten days when the tissue is stored at 4 degrees Celsius. The possibility of disease transmission requires careful donor selection [Pruitt, B A et al., Arch. Surg. 119, 312-322, (1984)].
Current developments in the field of allograft skin products focus on culturing epidermal cells to form skin-like coverings to be used as skin allografts as referenced in U.S. Pat. No. 5,015,584. Cryopreservation of allograft is commonly used, which retains the viability of the donor cells to some extent. It was previously believed that living cells were required for the success of skin allograft. However, good results have been obtained using methods which preserve the allograft without retaining the viability of the cells, such as preservation with glycerol [Kreis R W, et al., J. Trauma 29(1), 51-54 (1989)] [Hermans, M H E, Burns 15(1), 57-59 (1989)], silicone fluid [Ballantyne, D L Jr. et al., Cryobiology 8, 211-215, (1971)] or lyophilization [Young, J M et al., Arch. Surg. 80(Feb.), 208-213, (1960)].
Fresh-frozen allograft skin and lyophilized allograft skin have limitations such as demanding processing procedures. The requirements for such procedures confine the preparation of either material to special centers having proper facilities. The lyophilized material has an essentially unlimited nonrefrigerated shelf life, while the frozen material has a similarly prolonged shelf life provided proper refrigeration is maintained. Either material can be easily and rapidly prepared for use by rehydration or thawing. Lyophilized allograft skin generally adheres less well to the wound and is less able to reduce the bacterial count on the wound surface than fresh allograft skin [Pruitt, B A et al., Arch. Surg. 119, 312-322, (1984)].
U.S. Pat. Nos. 3,645,849 and 3,743,480 describe processes for of sterilization of biological material (e.g., blood serum) by microwave irradiation. Methods for preparing and sterilizing biological tissues such as heart valves, veins, cartilage, ligaments and organs for use as bioprostheses are described in U.S. Pat. No. 4,994,237. The source of irradiation is a microwave oven. This method tends to heat the specimen and destroy its structure. A method of sterilization of biological material by ultraviolet light is described in U.S. Pat. No. 4,880,512. Ultraviolet light is an efficient method of sterilization but it does not penetrate through objects such as skin very well. Consequently, this method is not always secure. In addition, Ultraviolet Light is not efficient for batch sterilization.
Another widely used method of biological tissue preservation and sterilization, which does not retain cell viability, is gamma irradiation. This method has been used extensively in the preservation of bone allograft, with good results. It has also been used in the preservation of donor cartilage [Dingman R O et al., Plast. Reconstr. Surg. 28(5), 562-567, (1961)], blood vessels, heart valves [Wright K A et al., Sterilization and Preservation of Biological Tissues by Ionizing Radiation. Vienna, International Atomic Energy Agency, 107-118, (1970)], dura mater, and sclera [Colvard D M et al., Am. J. Ophthal., 87(4), 494-496, (1979)]. Irradiation sterilization of the tissue permits storage at room temperature, a considerable advantage when low temperature storage is unavailable. U.S. Pat. No. 4,351,091 employs gamma and x-ray irradiation to preserve a corpse to kill bacteria and other microorganisms that contribute to the decomposition of a corpse. This patent does not address infectious diseases such as viruses or the feasibility of preparing or preserving the corpse for organ donation.
With the use of allograft skin, there is an associated risk of the transmission of disease, including the human immunodeficiency virus (HIV). Skin banks around the world were virtually closed down for two or more years after the reported transmission of HIV from allograft skin [Clarke J A, Lancet 1,983, (1987)]. Gamma irradiation at ranges of 250,000 cGy to 2.5 million cGy has been shown to inactivate HIV [Hiemstra H et al., Transfusion, 31(1), 32-39, (1991)] [Spire B et al., Lancet, 1, 188-189, (1985)]. The effect of gamma irradiation on human coagulation factors found in human plasma and on virus suspended in plasma or other types of suspending medium has been studied [Kitchen, A D et al., Vox Sang 56, 223-229, (1989)].
The present invention overcomes the above-described disadvantages inherent with various materials and methods of the prior art.